In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high device densities there have been, and continue to be, efforts toward scaling down device dimensions (e.g., at sub-micron levels) on semiconductor wafers. In order to accomplish such densities, smaller feature sizes and more precise feature shapes are required. This can include width and spacing of interconnecting lines, spacing and diameter of contact holes, and surface geometry, such as corners and edges, of various features. The dimensions of and between such small features can be referred to as critical dimensions (CDs). Reducing CDs and reproducing more accurate CDs facilitates achieving higher device densities.
High-resolution lithographic processes are used to achieve small features. In general, lithography refers to processes for pattern transfer between various media. In lithography for integrated circuit fabrication, a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the photoresist. The film is selectively exposed with radiation (e.g., optical light, x-ray, electron beam, . . . ) through an intervening master template (e.g., mask, reticle, . . . ) forming a particular pattern (e.g., patterned resist). Dependent upon coating type, exposed areas of the coating become either more or less soluble than unexposed areas in a particular solvent developer. More soluble areas are removed with the developer in a developing step, while less soluble areas remain on the silicon wafer to form a patterned coating. The pattern corresponds to either the image of the mask or its negative. The patterned resist is used in further processing of the silicon wafer.
The achievement of smaller critical dimensions is related to the resolution of the lithographic system. Efforts to increase resolution and thereby reduce critical dimensions can be accomplished by several approaches. One approach involves the reduction in wavelength of the exposure radiation such as is achieved by moving from mercury g-line (436 nm), to excimer laser (193 nm), and further to 157 nm, 90 nm, 65 nm, etc. A second approach involves improvements in optical designs, manufacturing techniques, and metrology. Such improvements have lead to increased resolution through an increase in numerical aperture. A third approach involves the utilization of various resolution enhancement techniques. The use of phase shifting masks and off-axis illumination techniques have led to improved resolution through a reduction in the lithographic constant “k” of the imaging system.
In low k imaging systems, a significant portion of the transmitted light energy is carried in high spatial frequency components of the mask spectrum. In general, these high spatial frequency components are not captured by the low-pass pupil of an imaging system. The loss of the high spatial frequency components results in images that are distorted in one or more ways from the original. Image distortion includes such effects as line shortening, corner rounding, non-linearity and proximity effects where, for example, imaged line width can vary as a function of spacing between adjacent lines. Image distortions can result in images that are of insufficient fidelity to satisfy their intended function. Methods to correct or offset such distortions can increase the process margin thereby improving efficiency of the system.
Optical proximity correction (OPC) is one particular methodology that can be employed to compensate for known pattern distortions. OPC is employed to compensate mask geometry for known effects that can occur during imaging. The utilization of OPC can provide improved line-width uniformity allowing for faster clock rates and better overall circuit performance. OPC can also enhance an imaging process window and thereby provide a higher yield. One OPC technique involves the addition of assist features to a mask that enable more consistent imaging of a desired pattern. However, the presence of assist features can result in the formation of an undesirable resist residue, which negatively impacts the yield of a lithographic process. Inhibiting the formation of resist residue can significantly improve the yield of a lithographic imaging process.